The present invention relates to a double-faced pressure-sensitive adhesive sheet and a pressure-sensitive adhesive member. More particularly, the invention relates to a double-faced pressure-sensitive adhesive sheet and a pressure-sensitive adhesive member which are suitable for use in applications such as the assembly of electronic appliances where contamination with a silicone should be avoided, e.g., magnetic recording apparatus (a hard disk drive (HDD)).
Pressure-sensitive adhesive sheets for use in the bonding of various articles have a release liner superposed on a surface of the pressure-sensitive adhesive layer for the purpose of protecting the pressure-sensitive adhesive layer until use. Generally used as such release liners are release sheets obtained by coating a base material, e.g., paper or a plastic film, with a silicone-series release agent.
On the other hand, double-faced adhesive sheets (tapes) among such pressure-sensitive adhesive sheets have recently come to be used also in the assembly of precision electronic parts (appliances) for, e.g., magnetic recording apparatus (HDD). In this case, a double-faced pressure-sensitive adhesive sheet is frequently used as a pressure-sensitive adhesive member obtained by stripping off one of the release liners with which both sides of the pressure-sensitive adhesive layer are covered and bonding a base material suitable for the intended use to the exposed surface of the pressure-sensitive adhesive layer.
In such applications, there is a desire to exclude any silicone-series material from the components of pressure-sensitive adhesive sheets in order to prevent contact failures and head crush troubles in magnetic disks. Among the components of pressure-sensitive adhesive sheets, those for which a silicone-series material is frequently used are release liners or the like. Namely, a silicone-series release agent is frequently used as a release agent for the release liners. Consequently, the most effective measure in satisfying that desire is not to use a silicone-series release agent.
Possible methods for avoiding the use of a silicone-series release agent include a method in which a fluorine-series release agent or a long-chain alkyl type release agent is used for release liners and a method in which polyolefin-series plastic films having poor adhesiveness, e.g., polyethylene or polypropylene, are used as release liners. However, use of a fluorine-series release agent in producing release liners to be used respectively for covering both sides of a double-faced adhesive sheet has a problem that the production cost is increased because the release agent is expensive. On the other hand, use of a long-chain alkyl type release agent in the above application has a drawback that the releasing force required for release liner stripping is higher than in the case of silicone-series release agents and this tends to result in reduced workability when one of the release liners is stripped off and the pressure-sensitive adhesive layer is applied to any of various base materials to produce a pressure-sensitive adhesive member. Furthermore, use of a polyolefin-series plastic film as a release liner tends to pose a problem concerning heat resistance, although it has been found that a release liner having low releasing force (excellent releasability) is obtainable depending on the composition of the plastic film. For example, in the case where a pressure-sensitive adhesive layer is formed on such a release liner through heat curing, there is a problem that the release liner curls during the heating, making it difficult to maintain the original sheet form or making it impossible to maintain the appearance quality of the double-faced adhesive sheet.
Consequently, an object of the invention is to provide a double-faced pressure-sensitive adhesive sheet which has excellent workability when one of the release liners of the double-faced adhesive sheet is stripped off and the exposed surface of the adhesive sheet is bonded to a base material to produce a pressure-sensitive adhesive member, and which does not cause silicone ingredient migration to adherends when used as the pressure-sensitive adhesive member in the assembly of, e.g., precision electronic parts for HDDs.
Another object of the invention is to provide a double-faced pressure-sensitive adhesive sheet which is prevented from curling even when produced through a heating step and which can be produced at low cost.
Still another object of the invention is to provide a pressure-sensitive adhesive member which can be easily and efficiently produced and does not cause silicone ingredient migration to adherends.
The present inventors made intensive studies in order to accomplish those objects. As a result, they have found that when a specific combination of release liners is used as the release liners respectively on both sides of a double-faced pressure-sensitive adhesive sheet, not only this double-faced adhesive sheet can have increased workability when used in producing a pressure-sensitive adhesive member therefrom, but also the pressure-sensitive adhesive member, even when used in the assembly of electronic parts or other products, does not cause the migration of a silicone ingredient to the electronic parts, etc. and thereby prevents contact failures or other troubles. The invention has thus been completed.
The invention provides a double-faced pressure-sensitive adhesive sheet comprising a pressure-sensitive adhesive layer which is covered on one side with a silicone-series release liner and on the other side with a non-silicone-series release liner. The non-silicone-series release liner can be constituted, for example, of a plastic film having a laminated structure composed of at least three layers.
The invention further provides a pressure-sensitive adhesive member obtained by stripping off the silicone-series release liner of the double-faced pressure-sensitive adhesive sheet and bonding a base material to the exposed surface of the pressure-sensitive adhesive layer. This pressure-sensitive adhesive member can be used as an adhesive member for assembling the hard disk drive of a computer.
The term xe2x80x9cadhesive sheetxe2x80x9d as used herein includes xe2x80x9cadhesive tapexe2x80x9d.